But, there are those last few casters.
Both Skarres can sort of run a few heavies, though they usually prefer the character heavies route as well.
pSkarre have between 7 and 12 focus a turn to play with, but because it isn't consistent, you can't reliably run a bunch of heavies with her. On a good turn, yes, she can go balls to the wall powering a bunch of jacks, but if things don't go as planned, or you get a 1 at the worst possible turn, she might struggle powering a large number of jacks.
eSkarre offers the greatest protective ability in the game with her feat, however it affects a maximum of 5 targets. 1 tending to be herself, this leaves you 4 things to get the love. But, she plays similarly to pDenny; she's going to want to be slinging spells at the same time to set off things like perdition, black spot, etc. So she, at her best, is still only wanting to handle so many heavies and preferably focus efficient ones.
HOWEVER; Thanks to seas of Fate, you can maximize your focus potential with her, so using the Power Boost ability from warwitch sirens (one of Cryx's BEST solos) you can potentially run an extra pair of heavies without losing TOO much efficiency.
eSkarre is probably the 2nd best 'Jack caster' of Cryx, though still being able to support infantry very well.